Method for encoding/decoding image and device using same

ABSTRACT

A method of decoding a bistream comprising a plurality of layers according to the present invention includes setting parameters for decoding the bitstream from a random access point when random access occurs, and decoding a decoding target picture, wherein the setting of the parameters includes setting an initialization flag indicating whether the decoding target picture, that is an intra random access point (IRAP) picture, is output by correctly decoding all reference layers of a target layer including the decoding target picture when the random access occurs; setting a flag variable (NoRaslOutputFlag) indicating whether the decoding target picture is used as a random access point; and resetting the initialization flag when the decoding target picture is an IRAP picture and the flag variable is 1.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/027,101 having a 371(c) date of Apr. 4, 2016, which is a U.S.National Stage Application of International Application No.PCT/KR2014/009409, filed on Oct. 7, 2014, which claims the benefit under35 USC 119(a) and 365(b) of Korean Patent Application No.10-2014-0134240, filed on Oct. 6, 2014, Korean Patent Application No.10-2014-0132543, filed on Oct. 1, 2014, Korean Patent Application No.10-2014-0031880, filed on Mar. 18, 2014, Korean Patent Application No.10-2013-0121357, filed on Oct. 11, 2013, and Korean Patent ApplicationNo. 10-2013-0121430, filed on Oct. 11, 2013 in the Korean IntellectualProperty Office, the entire disclosures of which are incorporated hereinby reference for all purposes.

TECHNICAL FIELD

The present invention relates to video encoding and decoding, and moreparticularly, to a method of encoding/decoding a video supporting randomaccess to a bitstream and an apparatus using the same.

BACKGROUND ART

In recent years, as high definition (HD) broadcast services arespreading domestically and globally, a large number of users are gettingused to high-resolution and high-quality videos and accordinglyinstitutions put spurs to the development of next-generation videodevices. Also, with growing interest in ultrahigh-definition (UHD)services having a resolution four times higher than HDTV, compressiontechniques for higher-quality videos are needed.

For video compression, there may be used an inter prediction techniqueof predicting pixel values included in a current picture from temporallyprevious and/or subsequent pictures of the current picture, an intraprediction technique of predicting pixel values included in a currentpicture using pixel information in the current picture, or an entropyencoding technique of assigning a short code to a symbol with a highappearance frequency and assigning a long code to a symbol with a lowappearance frequency.

Video compression technology may include a technique of providing aconstant network bandwidth in restricted operating environments ofhardware without considering variable network environments. However, tocompress video data used for network environments involving frequentchanges of bandwidths, new compression techniques are required, whereina scalable video encoding/decoding method may be employed.

DISCLOSURE Technical Problem

An aspect of the present invention is to provide a method for decoding abitstream from a random point and an apparatus using the same.

In detail, an aspect of the present invention a method of layer-wisestartup decoding a bitstream from a random access point when an accessunit (AU) at the random access point includes an intra random accesspoint (IRAP) picture and a non-IRAP picture, or includes only IRAPpictures which have two or more different network abstraction layer(NAL) unit types.

Technical Solution

An aspect of the present invention provides a method of decoding abistream including a plurality of layers, the method including settingparameters for decoding the bitstream from a random access point whenrandom access occurs; and decoding a decoding target picture, whereinthe setting of the parameters include setting an initialization flagindicating whether the decoding target picture is correctly decoded andoutput when the random access occurs; setting a flag variable(NoRaslOutputFlag) indicating whether the decoding target picture isused as a random access point; and resetting an initialization flag of atarget layer to 1 when the decoding target picture is an intra randomaccess point (IRAP) picture, the flag variable is 1, the initializationflag of the decoding target picture is 0 and initialization flags of allreference layers of the target layer are 1.

The setting of the initialization flag may set the initialization flagfor a base layer of an access unit (AU) at which the random accessoccurs to 1 and set the initialization flag for a higher layer than thebase layer to 0.

The setting of the flag variable (NoRaslOutputFlag) may set the flagvariable to 1 when the decoding target picture is an instantaneousdecoding refresh (IDR) picture, a broken link access (BLA) picture or anintra random access point (IRAP) picture which is a first picture of thetarget layer of the bitstream.

The setting of the flag variable (NoRaslOutputFlag) may set the flagvariable to 1 when the target picture is an IRAP picture, theinitialization flag of the target layer including the target picture is0, and the initialization flags of all reference layers of the targetlayer including the target picture are 1.

The method may further include calculating a picture order count (POC)of the decoding target picture as an identifier; constructing areference picture set and marking a reference picture type for interprediction of the decoding target picture; determining whether to outputthe decoding target picture based on the initialization flag; andperforming motion estimation and motion compensation on the decodingtarget picture using a reference picture in a reference picture listgenerated based on the reference picture set.

The determining of whether to output the decoding target picture maydetermine not to output the decoding target picture when the decodingtarget picture is an IRAP picture and the initialization flag of thetarget layer is 0.

The determining of whether to output the decoding target picture maydetermine to output the decoding target picture when the target layer isnot an output layer as an output target, an output target picturebelonging to the output layer in an AU which the target layer belongs tois determined not to be output, the target layer is used for the outputlayer for reference so that the decoding target picture is used as areference picture of the output target picture, and the decoding targetpicture is a highest picture among decoded pictures with flaginformation equal to 1, the flag information being signaled via a sliceheader and representing whether to output a picture.

The determining of whether to output the decoding target picture maydetermine to output the decoding target picture when the target layer isnot an output layer as an output target, an output target picturebelonging to the output layer in an AU which the target layer belongs tois determined not to be output, the target layer is used for the outputlayer for reference so that the decoding target picture is used as areference picture of the output target picture, the initialization flagof the target layer is 1, and the decoding target picture is a highestpicture among decoded pictures with flag information equal to 1, theflag information being signaled via a slice header and representingwhether to output a picture.

The method may further include decoding pictures included in a first AUcorresponding to the random access point; determining whether toindividually output the pictures in the first AU; decoding picturesincluded in a second AU following the first AU; and determining whetherto individually output the pictures in the second AU.

The method may further include determining outputting a first AU picturewhich outputs a picture determined to be output among the pictures inthe first AU, wherein the outputting of the first AU picture outputs anoutput picture of an output layer when the bitstream includes a baselayer to a highest layer, all pictures from base layer to an nth layer(0≤n) are IRAP pictures, and the base layer to the nth layer include theoutput layer, and outputs a decoded picture in the nth layer when thebitstream includes a base layer to a highest layer, and an (n+1)th layerto the highest layer include the output layer.

The method may further include determining outputting a second AUpicture which outputs a picture determined to be output among thepictures in the second AU, wherein the outputting of the second AUpicture outputs an output picture of an output layer when the (n+1)thlayer to a kth layer (n+1≤k) included in the second AU include an IRAPpicture and the base layer to the kth layer include the output layer,and outputs a decoded picture in the kth layer or the picture determinedto be output when a (k+1) layer to the highest layer comprise the outputlayer.

The calculating of the POC may set the POC of the decoding targetpicture to 0 when a picture of a base layer in the same AU is an IDRpicture, and set a POC_most significant bit (MSB) of the decoding targetpicture to 0 when the picture of the base layer is a BLA picture or afirst CRA picture of the bitstream.

Another aspect of the present invention provides an apparatus fordecoding a bistream including a plurality of layers, the apparatusincluding: a prediction module to set a parameter for decoding thebitstream from a random access point when random access occurs and todecode a decoding target picture, wherein the prediction module sets aninitialization flag indicating whether the decoding target picture iscorrectly decoded and output when the random access occurs, sets a flagvariable (NoRaslOutputFlag) indicating whether the decoding targetpicture is used as a random access point, and resets an initializationflag of a target layer to 1 when the decoding target picture is an IRAPpicture, the flag variable is 1, the initialization flag of the decodingtarget picture is 0 and initialization flags of all reference layers ofthe target layer are 1.

Advantageous Effects

According to an embodiment of the present invention, there are provideda method for decoding a bitstream from a random point and an apparatususing the same.

In detail, according to an embodiment of the present invention, thereare provided a method of decoding a bitstream from a random access pointwhen an AU at the random access point includes an IRAP picture and anon-IRAP picture, or includes only IRAP pictures which have two or moredifferent NAL unit types, and an apparatus using the same.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a videoencoding apparatus according to an embodiment.

FIG. 2 is a block diagram illustrating a configuration of a videodecoding apparatus according to an embodiment.

FIG. 3 is a conceptual diagram schematically illustrating a scalablevideo coding structure using a plurality of layers according to anembodiment of the present invention.

FIG. 4 illustrates a method of layer-wise startup decoding a bitstreamaccording to the present invention.

FIG. 5 illustrates a bitstream structure including a random access pointaccording to an embodiment of the present invention.

FIG. 6 illustrates a bitstream structure including a random access pointaccording to another embodiment of the present invention.

FIG. 7 illustrates a bitstream structure including a random access pointaccording to still another embodiment of the present invention.

FIG. 8 illustrates a bitstream structure including a random access pointaccording to yet another embodiment of the present invention.

FIG. 9 is a flowchart illustrating a video decoding method according toan embodiment of the present invention.

FIG. 10 illustrates a method of setting decoding parameters according toan embodiment of the present invention.

FIG. 11 is a flowchart illustrating a method of decoding an inter-layervideo according to an embodiment of the present invention.

MODE FOR INVENTION

Hereinafter, embodiments of the present invention are described indetail with reference to the accompanying drawings. In describing theembodiments of the present invention, a detailed description of relatedknown elements or functions will be omitted if it is deemed to make thegist of the present invention unnecessarily vague.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, the element can be directlyconnected or coupled to another element or intervening elements. Also,when it is said that a specific element is “included,” it may mean thatelements other than the specific element are not excluded and thatadditional elements may be included in the embodiments of the presentinvention or the scope of the technical spirit of the present invention.

Although the terms “first,” “second,” etc. may be used to describevarious elements, these elements should not be limited by these terms.These terms are used only to distinguish one element from anotherelement. For example, a first element may be named a second elementwithout departing from the scope of the present invention. Likewise, asecond element may be named a first element.

Although components described in the embodiments of the presentinvention are independently illustrated in order to show differentcharacteristic functions, such a configuration does not indicate thateach component is constructed by a separate hardware constituent unit orsoftware constituent unit. That is, each component includes individualcomponents that are arranged for convenience of description, in which atleast two components may be combined into a single component or a singlecomponent may be divided into a plurality of components to performfunctions. It is to be noted that embodiments in which some componentsare integrated into one combined component and/or a component is dividedinto multiple separate components are included in the scope of thepresent invention without departing from the essence of the presentinvention.

Some constituent elements are not essential to perform the substantialfunctions in the invention and may be optional constituent elements formerely improving performance The present invention may be embodied byincluding only constituent elements essential to implement the spirit ofthe invention other than constituent elements used for merely improvingperformance. A structure including only the essential constituentelements other than optional constituents used for merely improvingperformance also belongs to the scope of the present invention.

FIG. 1 is a block diagram illustrating a configuration of a videoencoding apparatus according to an embodiment. A scalable videoencoding/decoding method or apparatus may be realized by extension of ageneral video encoding/decoding method or apparatus that does notprovide scalability, and the block diagram of FIG. 1 illustrates anexample of a video encoding apparatus which may form a basis for ascalable video encoding apparatus.

Referring to FIG. 1, the video encoding apparatus 100 includes a motionestimation module 111, a motion compensation module 112, an intraprediction module 120, a switch 115, a subtractor 125, a transformmodule 130, a quantization module 140, an entropy encoding module 150,an dequantization module 160, an inverse transform module 170, an adder175, a filter module 180, and a reference picture buffer 190.

The video encoding apparatus 100 may encode an input picture images inan intra mode or an inter mode and output a bitstream. Intra predictionmeans an intra-picture prediction, and inter prediction means aninter-picture prediction. In the intra mode, the switch 115 is shiftedto ‘intra,’ and in the inter mode, the switch 115 is shifted to ‘inter.’The video encoding apparatus 100 may generate a prediction block for aninput block of the input picture and then encode a difference betweenthe input block and the prediction block.

In the intra mode, the intra prediction module 120 may perform spatialprediction by using a pixel value of a pre-encoded block around acurrent block to generate a prediction block.

In the inter mode, the motion estimation module 111 may obtain a regionwhich is most matched with the input block in the reference picturestored in the reference picture buffer 190 during a motion estimationprocess to derive a motion vector. The motion compensation module 112may perform motion compensation using the motion vector and thereference picture stored in the reference picture buffer 190, therebygenerating the prediction block.

The subtractor 125 may generate a residual block based on the differencebetween the input block and the generated prediction block. Thetransform module 130 may transform the residual block to output atransform coefficient. The quantization module 140 may quantize thetransform coefficient according to a quantization parameter to output aquantized coefficient.

The entropy encoding module 150 may entropy-encode a symbol according toprobability distribution based on values derived by the quantizationmodule 140 or an encoding parameter value derived in encoding, therebyoutputting a bitstream. Entropy encoding is a method of receivingsymbols having different values and representing the symbols as adecodable binary sequence or string while removing statisticalredundancy.

Here, a symbol means a syntax element as an encoding/decoding target, acoding parameter, a value of a residual signal, or the like. A codingparameter, which is a parameter necessary for encoding and decoding, mayinclude information encoded by the encoding apparatus and transferred tothe decoding apparatus, such as a syntax element, and information to beinferred during an encoding or decoding process and means informationnecessary for encoding and decoding a picture. The coding parameter mayinclude, for example, values or statistics of an intra/inter predictionmode, a movement/motion vector, a reference picture index, a codingblock pattern, presence and absence of a residual signal, a transformcoefficient, a quantized transform coefficient, a block size and blockpartition information. A residual signal may denote a difference betweenan original signal and a prediction signal, a transformed signal of thedifference between the original signal and the prediction signal, or atransformed and quantized signal of the difference between the originalsignal and the prediction signal. The residual signal may be referred toas a residual block in a block unit.

When entropy encoding is applied, a symbol having a high probability isallocated a small number of bits and a symbol having a low probabilityis allocated a large number of bits in representation of symbols,thereby reducing a size of bit strings for symbols to be encoded.Accordingly, entropy encoding may enhance compression performance ofvideo encoding.

For entropy encoding, encoding methods, such as exponential Golomb,context-adaptive variable length coding (CAVLC) and context-adaptivebinary arithmetic coding (CABAC), may be used. For example, a table usedfor performing entropy encoding, such as a variable length coding/code(VLC) table, may be stored in the entropy encoding module 150, and theentropy encoding module 150 may perform entropy encoding using thestored VLC table. In addition, the entropy encoding module 150 mayderive a binarization method of a target symbol and a probability modelof a target symbol/bin and perform entropy encoding using the derivedbinarization method or probability model.

The quantized coefficient may be dequantized by the dequantizationmodule 160 and inversely transformed by the inverse transform module170. The dequantized and inversely transformed coefficient is added tothe prediction block by the adder 175, thereby generating areconstructed block.

The reconstructed block is subjected to the filter module 180, and thefilter module 180 may apply at least one of a deblocking filter, asample adaptive offset (SAO), and an adaptive loop filter (ALF) to thereconstructed block or a reconstructed picture. The reconstructed blockobtained via the filter module 180 may be stored in the referencepicture buffer 190.

FIG. 2 is a block diagram illustrating a configuration of a videodecoding apparatus according to an embodiment. As described above inFIG. 1, a scalable video encoding/decoding method or apparatus may berealized by extension of a general video encoding/decoding method orapparatus that does not provide scalability, and the block diagram ofFIG. 2 illustrates an example of a video decoding apparatus which mayform a basis for a scalable video decoding apparatus.

Referring to FIG. 2, the video decoding apparatus 200 includes anentropy decoding module 210, a dequantization module 220, an inversetransform module 230, an intra prediction module 240, a motioncompensation module 250, a filter module 260, and a reference picturebuffer 270.

The video decoding apparatus 200 receives an input bitstream output fromthe encoding apparatus and decodes the bitstream in an intra mode orinter mode to output a reconstituted picture, that is, a reconstructedpicture. In the intra mode, a switch may be shifted to ‘intra,’ and inthe inter mode, the switch may be shifted to ‘inter. The video decodingapparatus 200 may obtain a residual block reconstructed from the inputbitstream, generate a prediction block, and add the residual block andthe prediction block to generate a reconstituted block, that is, areconstructed block.

The entropy decoding module 210 may entropy-decode the input bitstreamaccording to probability distribution to generate symbols including asymbol in a form of a quantized coefficient. Entropy decoding is amethod of receiving a binary sequence to generate symbols. The entropydecoding method is similar to the aforementioned entropy encodingmethod.

The quantized coefficient is dequantized by the dequantization module220 and inversely transformed by the inverse transform module 230,thereby generating a reconstructed residual block.

In the intra mode, the intra prediction module 240 may perform spatialprediction by using a pixel value of a pre-encoded block around acurrent block to generate a prediction block. In the inter mode, themotion compensation module 250 may perform motion compensation using amotion vector and a reference picture stored in the reference picturebuffer 270, thereby generating a prediction block.

The reconstructed residual block and the prediction block are added byan adder 255, and the added blocks are subjected to the filter module260. The filter module 260 may apply at least one of a deblockingfilter, an SAO, and an ALF to the reconstructed block or thereconstructed picture. The filter module 260 outputs the reconstitutedpicture, that is, the reconstructed picture. The reconstructed picturemay be stored in the reference picture buffer 270 to be used for interprediction.

Among the entropy decoding module 210, the dequantization module 220,the inverse transform module 230, the intra prediction module 240, themotion compensation module 250, the filter module 260 and the referencepicture buffer 270 of the decoding apparatus 200, components directlyrelated to video decoding, for example, the entropy decoding module 210,the dequantization module 220, the inverse transform module 230, theintra prediction module 240, the motion compensation module 250 and thefilter module 260 may be defined as a decoder or a decoding unit,separately from the other components.

Further, the decoding apparatus 200 may further include a parsing module(not shown) to parse information about an encoded video included in thebitstream. The parsing module may include the entropy decoding module210 or be included in the entropy decoding module 210. The parsingmodule may be provided as one component of the decoding unit.

FIG. 3 is a conceptual diagram schematically illustrating a scalablevideo coding structure using a plurality of layers according to anembodiment of the present invention. In FIG. 3, Group of Picture (GOP)denotes a picture group, that is, a group of pictures.

In order to transmit video data, a transmission medium is needed, andperformance thereof is different by each transmission medium accordingto various network environments. For application to various transmissionmedia or network environments, a scalable video coding method may beprovided.

The scalable video coding method is a coding method which utilizestexture information, motion information, residual signals betweenlayers, or the like to remove redundancy between layers, thus improvingencoding and decoding performance. The scalable video coding method mayprovide various scalabilities in spatial, temporal, and quality aspectsaccording to ambient conditions such as transmission bit rate,transmission error rate, and system resources.

Scalable video coding may be performed by using a multi-layer structureso as to provide a bitstream applicable to various network situations.For example, the scalable video coding structure may include a baselayer in which video data is compressed and processed using a generalvideo decoding method, and also include an enhancement layer in whichvideo data is compressed and processed using both decoding informationof the base layer and a general video decoding method.

Here, a layer refers to a set of pictures and bitstreams that areclassified according to a spatial aspect (for example, picture size), atemporal aspect (for example, encoding order, picture output order andframe rate), picture quality, complexity, or the like. Further, the baselayer may mean a lower layer, a reference layer or a basic layer, andthe enhancement layer may mean a higher layer or an enhancing layer. Aplurality of layers may have dependency on each other.

Referring to FIG. 3, for example, the base layer may be defined bystandard definition (SD), 15 Hz frame rate and 1 Mbps bit rate, a firstenhancement layer may be defined by high definition (HD), 30 Hz framerate and 3.9 Mbps bit rate, and a second enhancement layer may bedefined by 4K-ultra high definition (UHD), 60 Hz frame rate and 27.2Mbps. These formats, frame rates and bit rates are provided only forillustrative purposes and may be changed and modified as needed. Also, anumber of used layers may change depending on circumstances, withoutbeing limited to the present embodiment.

For instance, when a transmission bandwidth is 4 Mbps, the firstenhancement layer HD may be transmitted at a frame rate reduced to 15 Hzor lower. The scalable video coding method may provide spatial,temporal, and quality scalabilities using the method described abovewith reference to FIG. 3.

In video encoding and decoding supporting a plurality of layers in abitstream, that is, scalable coding, since the layers have strongcorrelations, prediction is performed using such correlations, therebyremoving redundant elements of data and enhancing video encodingperformance. Hereinafter, prediction of a current layer as a predictiontarget using information on another layer is referred to as inter-layerprediction. Scalable video coding may refer to scalable video encodingin encoding, and to scalable video decoding in decoding.

The plurality of layers may be different from each other in terms of atleast one of resolution, frame rate and color format, and upsampling ordownsampling may be performed on a layer to adjust resolution ininter-layer prediction.

Random access in scalable video coding (SVC) will be describedconsidering as follows.

A bitstream may support at least one scalability, for example spatial,quality or temporal scalability, and a video having a structure of atleast one or more layers may be encoded/decoded such that random accessis possible.

A plurality of layers, for example, one or more layers, may be provided,wherein a lowest layer may be referred to as a base layer. Three layersare illustrated in the following description. However, the presentinvention is applicable without limiting the number of layers.

A higher layer and a lower layer may be encoded/decoded by a single loopor multiple loop method.

When a random access point of a bitstream is generated in the encodingapparatus, the decoding apparatus may decode the bitstream from therandom access point generated by the encoding apparatus.

A base layer included in an access unit (AU) at a random access pointmay be encoded as an intra random access point (IRAP) picture, andhigher layers may be encoded as either non-IRAP pictures or IRAPpictures. An AU refers to a set of Network Abstraction Layer (NAL) unitsor pictures having the same output time.

Meanwhile, only when all pictures of layers at a random access point areencoded into randomly accessible forms, decoding a bitstream from thepoint is possible. That is, when a randomly inaccessible picture isincluded in layers at a random access point, it is impossible to decodea bitstream from the point.

The present invention proposes a method for decoding a bitstream from arandom access point even when pictures of layers at the random accesspoint include a randomly inaccessible picture.

FIG. 4 illustrates a method of layer-wise startup decoding a bitstreamaccording to the present invention.

As shown in FIG. 4, there are three layers (layer 0, layer 1 and layer2), each of which may include an IRAP picture and a non-IRAP picture.

Among pictures in AU A, a picture in a first layer (layer 0) is an IRAPpicture, while pictures in a second layer (layer 1) and a third layer(layer 2) are non-IRAP pictures. Meanwhile, among pictures in AU B, apicture in the second layer (layer 2) is an IRAP picture, while picturesin the first layer (layer 0) and the third layer (layer 2) are non-IRAPpictures. Among pictures in AU C, a picture of the third layer (layer 2)is an IRAP picture, while pictures of the first layer (layer 0) and thesecond layer (layer 1) are non-IRAP pictures.

In the present invention, a bitstream may be encoded/decoded such thatpictures are displayed gradually from a correctly decoded lower layer toa highest layer to finally display as displayed in FIG. 4.

FIGS. 5 to 8 illustrate a bitstream structure including a random accesspoint according to embodiments of the present invention.

In FIG. 5, as in FIG. 4, each AU includes a single IRAP picture, whereinlowest to higher layers temporally sequentially include IRAP pictures.In a bitstream of FIG. 5, a first layer (layer 0) may be decoded from AUA, a first layer (layer 0) and a second layer (layer 1) may be decodedfrom AU B, and all layers (layer 0, layer 1 and layer 2) may be decodedfrom AU C.

In FIG. 6, pictures in a first layer (layer 0) and a second layer(layer 1) are IRAP pictures among pictures included in AU A, and only apicture of the second layer (layer 1) is an IRAP picture in AU B.

In a bitstream of FIG. 6, the first layer (layer 0) and the second layer(layer 1) may be decoded from AU A, and from the first to third layers,that is, all layers (layer 0, layer 1 and layer 3) may be decoded fromAU C.

In FIG. 7, among pictures included in AU A, a picture in a first layer(layer 0) is an IRAP picture and pictures in a second layer (layer 1)and a third layer (layer 2) are non-IRAP pictures. Among picturesincluded in AU B, pictures in the first layer (layer 0) and the secondlayer (layer 1) are IRAP pictures, and among pictures included in AU C,pictures in the second layer (layer 1) and the third layer (layer 2) areIRAP pictures.

In FIG. 7, as in FIG. 5, the first layer (layer 0) may be decoded fromAU A, the first layer (layer 0) and the second layer (layer 1) may bedecoded from AU B, and all layers (layer 0, layer 1 and layer 2) may bedecoded from AU C.

In FIG. 8, among pictures included in AU A, only a picture in a firstlayer (layer 0) is an IRAP picture and pictures in a second layer(layer 1) and a third layer (layer 2) are non-IRAP pictures. Amongpictures included in AU B, pictures in the first layer (layer 0) and thethird layer (layer 2) are IRAP pictures, and among pictures included inAU C, only a picture in the second layer (layer 1) is an TRAP picture.In AU D, only a picture in the third layer (layer 2) is an IRAP picture.

In FIG. 8, the first layer (layer 0) may be decoded from AU A, and in AUB, the picture in the second layer (layer 1) is not an IRAP picture buta non-IRAP picture and thus is not decoded. In this case, pictures inthe third layer (layer 2) which refer to the second layer (layer 1) maynot be output normally Thus, the first layer (layer 0) and the secondlayer (layer 1) may be decoded from AU C, then, all layers (layer 0,layer 1 and layer 2) may be decoded from AU D including an IRAP picturein the second layer (layer 1).

The present invention provides an encoding and decoding method whichenables random access from AU A in the bitstreams with the structures ofFIGS. 5 to 8. However, the encoding and decoding method according to thepresent invention is not restrictively applied to the bitstreamstructures of FIGS. 5 to 8.

In decoding a bitstream from a random access point, when an AU having abase layer that is an IRAP picture and having a higher layer is a firstAU in the bitstream, a process of encoding/decoding the bitstream fromthe AU is illustrated as follows.

A method of decoding a bitstream from a random access point according tothe present invention may be also applied to a case where a bitstream isinput and decoded up to an nth layer and then layers start to be inputup to an (n+k)th layer (here, k>0), that is, layer-switching.

When layer-wise startup is initiated from a random access point, abitstream may have such a structure that an nth layer has a shorter IRAPpicture period than an (n+1)th layer (0<=n<=62).

FIG. 9 is a flowchart illustrating a video decoding method according toan embodiment of the present invention. Details mentioned with referenceto FIG. 9 may be applied to not only the video decoding method but alsoa video encoding method.

First, the video decoding apparatus may set parameters needed fordecoding (S910).

When a bitstream is decoded from a random access point, the parametersneeded for decoding may be initialized or reset according to thefollowing process. An order of (4) and (5) do not change. In detail,initialization process (5) is conducted after (3) and (4), and may beperformed only when NoRaslOutputFlag is 1 in (3) or (4).

(1) When a base layer included in an AU at a random access point is anIRAP picture, that is, an instantaneous decoding refresh (IDR) picture,a broken link access (BLA) picture or a clean random access (CRA)picture, a flag indicating that all pictures included in the AU at therandom access point are a first picture of each layer included in thebitstream, for example, FirstPiclnLayerDecodedFlag, may be initializedas follows.

When a current encoding/decoding target picture is an IRAP picture whichis included in the base layer (nuh_layer_id=0) in the AU at the randomaccess point, flags of all possible higher layers,FirstPiclnLayerDecodedFlag, may be initialized to 0.

That is, when a maximum number of possible higher layers is, forexample, 63, that is, nuh_layer_id=63, FirstPicInLayerDecodedFlag[i],where 0<i<=63, may be set to 0.

(2) When the bitstream in which the base layer included in the AU at therandom access point is the IRAP picture (that is, IDR picture, BLApicture or CRA picture) is decoded from the random access point, aninitialization flag (for example, referred to as LayerinitialisedFlag)indicating that the corresponding layer among layers included in thebitstream is an IRAP picture and all reference layers of the layer arenormally initialized so that a decoded picture of the layer is used asan output of the decoding apparatus may be set as follows. That is, theinitialization flag indicates whether the decoding target picture iscorrectly decoded and output.

When a current encoding/decoding target picture is an IRAP picture andincluded in the base layer (nuh_layer_id=0) included in the AU at therandom access point, LayerInitialisedFlag[0] is set to 1.LayerInitialisedFlag[i] of the other layers (1<=i<=63) is set to 0.

The initialization flag may be set to either 0 or 1 for each individuallayer when decoding the bitstream starts, and then be reset depending ona flag variable and whether a reference layer referenced by acorresponding layer is correctly decoded, which will be mentioned below.

(3) When a current encoding/decoding target picture is an IDR picture, aBLA picture or an IRAP picture (including a CRA), which is a firstpicture of a layer (having the same nuh_layer_id as a layer of thecurrent encoding/decoding target picture) in the bitstream,NoRaslOutputFlag may be set to 1. NoRaslOutputFlag is a flag variableindicating whether the IRAP picture is used as a random access point.

(4) When a current encoding/decoding target picture included in an AUdecoded after the random access point in decoding order is an IRAPpicture (IDR, BLA or CRA picture), nuh_layer_id=n,LayerInitialisedFlag[n] is 0, and all lower reference layers of a layerincluding the current encoding/decoding target picture are normallyinitialized (that is, LayerInitialisedFlag[RefLayerId[n][j]]=1, whereRefLayerId[n][j] is nuh_layer_id of a jth reference layer of the currentencoding/decoding target picture in an nth higher layer(nuh_layer_id=n)), NoRaslOutputFlag may be set to 1.

That is, when the decoding target picture is an IRAP picture, aninitialization flag of a layer including the decoding target picture isset to 0, and an initialization flag of all reference layers of thelayer including the decoding target picture is 1, a flag variable is setto 1.

(5) When a current encoding/decoding target picture is an IRAP picturein an nth higher layer (nuh_layer_id=n), not a picture of a base layer(nuh_layer_id=0), has NoRaslOutputFlag equal to 1, and hasLayerInitialisedFlag[n] equal to 0, if all lower reference layers of alayer including the current encoding/decoding target picture arenormally initialized (that is, LayerInitialisedFlag[RefLayerId[n][j] is1, where RefLayerId[n][j] refers to nuh_layer_id of a jth referencelayer of the current encoding/decoding target picture in the nth higherlayer (nuh_layer_id=n)), LayerInitialisedFlag[n] may be set to 1.

That is, when the decoding target picture is an IRAP picture, a flagvariable is 1, an initialization flag of the decoding target picture isset to 0, and an initialization flag of all reference layers of thelayer including the decoding target picture is 1, the initializationflag of the target layer is reset to 1 from 0.

The decoding apparatus calculates an identifier, Picture Order Count(POC), of the current encoding/decoding target picture, a POC valueincreasing according to output order (S920).

A POC is an identifier to identify pictures of a layer having the samenuh_layer_id in a coded video stream, and a later picture output from adecoded picture buffer (DPB) has an increasing POC. That is, a POC is adisplay order of a picture output from the DPB and displayed, and an IDRpicture may have a POC of 0.

All pictures in the same AU may have the same POC.

When the same AU does not include an IRAP picture and a non-IRAP picturetogether, or includes only IRAP pictures which have the same NAL unittype, a POC may be calculated by a general method as follows.

A POC includes POC_most significant bit (MSB) and POC_(—) leastsignificant bit (LSB), and an overall POC may be a sum of an MSB and anLSB (POC=POC_MSB+POC_LSB). Here, a POC_LSB value is signaled via a sliceheader of a picture, and a maximum LSB, MaxPOCLSB, may be signaled via asequence parameter set.

POC_MSB of a non-IRAP picture may be calculated using POC_MSB(“prevPOCMSB”) and POC_LSB (“prevPOCLSB”) of a picture close to thecurrent picture among pictures having temporal_id of 0 and POC_LSB ofthe current encoding/decoding target picture, temporal_id representingan identifier of a temporal sub-layer previously encoded/decoded.

POC of an IDR picture may be always assumed to be 0. POC_MSB of a CRApicture being a first picture of the bitstream or of a BLA picture isassumed to be 0, and POC_LSB thereof may be signaled via a slice header.When a CRA picture is not the first picture of the bitstream, a POC maybe calculated in the same manner as the non-IRAP picture.

When the same AU includes an IRAP picture and a non-IRAP picturetogether or includes only IRAP pictures which have two or more differentNAL unit types, the pictures may have the same POC. To this end, flaginformation, such as poc_reset_flag, may be signaled via a slice header.The decoding apparatus may set POCs of all pictures in the AU includingthe slice to 0 when poc_reset_flag is 1. Here, the decoding apparatusmay calculate an original POC value (POC 1) of the currentencoding/decoding target picture by a general method, decrease POCs ofreference layers of the layer including the picture present in the DPBby POC 1, and set the POC of the current target picture to 0.

Hereinafter, calculating a POC of a picture will be described in detailwith reference to the following two cases.

First Case

When the bitstream including previously encoded AUs is decoded with anAU not being cut, that is, not being used as a random access point,pictures included in the AU may be decoded by the following process.

-   -   (a) An initial POC of the current encoding/decoding target        picture may be set as follows.

When the picture is an IDR picture, the POC may be set to 0.

When the picture is a BLA picture or a CRA picture that is a firstpicture of a corresponding layer of the bitstream, POC_MSB is 0 and thePOC may be set to POC_LSB signaled via a slice header.

When the picture is a CRA picture or a non-IRAP picture, the POC (POC 1)may be set by the general POC setting method using POC_LSB signaled viathe slice header (as mentioned above, using POC_MSB (prevPOCMSB) andPOC_LSB (prevPOCLSB) of a picture close to the current picture amongpreviously encoded/decoded pictures with temporal_id of 0 and POC_LSB ofthe current encoding/decoding target picture).

(b) When pictures including a slice with poc_reset_flag equal to 1 arepresent in the DPB, POCs of all pictures in the DPB may be reduced byPOC1.

(c) A POC of a picture including a slice with poc_reset_flag equal to 1may be set to 0.

Second Case

When an AU is cut and used as a random access point, that is, when theAU is decoded as a first AU of the bitstream, for example, when apicture in the base layer (layer 0) is an IRAP picture which is a firstpicture of the encoded bitstream or a BLA picture, pictures included inthe AU may be decoded by the following process.

(a) An initial POC of the current encoding/decoding target picture maybe set as follows.

When the picture is an IDR picture, the POC may be set to 0.

When the picture is a BLA picture or a CRA picture that is a firstpicture of a corresponding layer of the bitstream, POC_MSB is 0 and thePOC may be set to POC_LSB signaled via a slice header.

When the picture is a non-IRAP picture (for example, the current layerhas nuh_layer_id=i and FirstPicInLayerDecodedFlag[i]=0), the POC (POC1)may be set by one of the following methods.

(a-1) The POC (POC1) may be set by the aforementioned general POCsetting method using POC_LSB signaled via the slice header. Here, as thecurrent AU is used as a random access point, there are no previouslyencoded/decoded pictures with temporal_id equal to 0, and thus bothprevPOCMSB and prevPOCLSB may be set to 0.

(a-2) When a BLA or CRA picture is present in a base layer of the sameAU, POC_MSB of the current decoding target picture may be set to 0. Inthis case, the POC (POC1) of the current decoding target picture is setto POC_LSB signaled via the slice header of the current picture.

(a-3) The POC of the current decoding target picture may not becalculated.

(a-4) When an IDR picture is present in the base layer of the same AU,the POC of the current decoding target picture may be set to 0, withoutbeing calculated.

(b) After the POC of the current decoding target picture is calculated,when pictures including a slice with poc_reset_flag equal to 1 arepresent in the DPB, POCs of all pictures in the DPB may be reduced byPOC1.

(c) A POC of a picture including a slice with poc_reset_flag equal to 1may be set to 0.

Next, the video decoding apparatus constructs a reference picture setand conducts reference picture marking for inter prediction of thecurrent encoding/decoding target picture (S930).

When poc_reset_flag initializing a POC is 0, POCs or POC_LSBs of slicereference pictures of the picture may be calculated as follows.

(1) POCs of short-term reference pictures (STRPs) are calculated usingdelta_POC representing each STRP signaled via a corresponding sliceheader and the POC of the current picture, POC1.

Here, delta_POC may be a POC difference between the current picture andan ith STRP or a POC difference between an (i+1)th STRP and an ith STRP.

(2) POC_LSBs or POCs of long-term reference pictures (LTRPs) arecalculated using POC_LSB of each LTRP, delta_poc_msb_cycle_lt forcalculating POC_MSB, and POC1 as the POC of the current picture.delta_poc_msb_cycle_lt may be signaled via a corresponding slice header.

For an LTRP, only POC_LSB is basically signaled and a POC of the LTRP isidentified just by signaled POC_LSB. Here, when there is a referencepicture having the same POC_LSB as that of a corresponding LTRP among aplurality of reference pictures, delta_poc_msb_cycle_lt for calculatingPOC_MSB is additionally signaled, thereby identifying a POC of eachreference picture.

Meanwhile, when poc_reset_flag is 1, POCs or POC_LSBs of slice referencepictures of the picture may be calculated as follows.

(1) POCs of STRPs are calculated using delta_POC representing each STRPsignaled via a corresponding slice header and the POC of the currentpicture equal to 0.

Here, delta_POC may be a POC difference between the current picture andan ith STRP or a POC difference between an (i+1)th STRP and an ith STRP.

(2) POC_LSBs or POCs of LTRPs are calculated using POC_LSB of each LTRP,delta_poc_msb_cycle_lt for calculating POC_MSB, the POC of the currentpicture equal to 0, and POC_LSB signaled via a slice header of thecurrent picture. delta_poc_msb_cycle_lt may be signaled via a sliceheader of a corresponding LTRP.

For an LTRP, only POC_LSB is basically signaled and a POC of the LTRP isidentified just by signaled POC_LSB. Here, when there is a referencepicture having the same POC_LSB as that of a corresponding LTRP among aplurality of reference pictures, delta_poc_msb_cycle_lt for calculatingPOC_MSB is additionally signaled, thereby identifying a POC of eachreference picture.

As such, when poc_reset_flag is 1, POCs or POC_LABs of referencepictures signaled via a slice header of the picture including the slicewith poc_reset_flag equal to 1 may be adjusted using POC1 of the currentpicture to correspond to the reduced POCs of the pictures in the DPB.

When the POCs of the reference pictures are calculated, a referencepicture set is constructed and a type of a reference picture may bemarked depending on whether the reference picture is present in the DPB.

The video decoding apparatus may generate a virtual reference picturewhich is included in the reference picture set but is absent in the DPB(S940).

When nuh_layer_id=0, the current encoding/decoding target picture is anIRAP picture and NoRaslOutputFlag=1 or when nuh_layer_id>0 (when thecurrent encoding/decoding target picture is an IRAP picture withFirstPicInLayerDecodedFlag[nuh_layer_id]=1 and NoRaslOutputFlag=1 orwhen FirstPicInLayerDecodedFlag[nuh_layer_id]=0), pictures absent in theDPB among pictures included in the reference picture set for the currentencoding/decoding target picture (constructed in S930) may be virtuallygenerated and stored in the DPB.

The generated virtual pictures have the same POCs as those of thereference pictures calculated in S920. PicOutputFlag indicating whethera virtual reference picture is output from the DPB is set to 0, at whichthe virtual reference picture is not output.

The generated virtual reference pictures may be marked as either an STRPor an LTRP as signaled in the reference picture set. In detail, thevirtual reference pictures may be marked as an STRP which is referencedby the current picture and precedes the current picture in displayorder, an STRP which is referenced by the current picture and followsthe current picture in display order, an LTRP referenced by the currentpicture, an STRP referenced by subsequent pictures to the currentpicture, or an LTRP referenced by subsequent pictures to the currentpicture.

Next, the video decoding apparatus may determine whether to output thecurrent encoding/decoding target picture (S950). PicOutputFlagindicating whether to output a decoded picture may be set as follows.When PicOutputFlag is 1, the picture is sent from the DPB as an outputof the decoding apparatus. When PicOutputFlag is 0, the picture may notbe sent from the DPB as an output of the decoding apparatus.

(1) When the current picture is a random access skipped leading (RASL)picture and NoRaslOutputFlag of an IRAP picture closest to the currentpicture among previously encoded/decoded IRAP pictures to the currentpicture is 1, PicOutputFlag may be set to 0.

Here, an RASL picture is a leading picture following the IRAP pictureclosest to the current picture among the previously encoded/decoded IRAPpictures to the current picture in encoding/decoding order but precedingthe IRAP picture in display order, and is encoded/decoded using anencoded/decoded picture prior to the IRAP picture as a referencepicture.

(2) When condition (1) is not satisfied, the picture has nuh_layer_idequal to n and LayerInitialisedFlag[n]=0, PicOutputFlag may be set to 0.

(3) When conditions (1) and (2) are not satisfied, PicOutputFlag may beset by one of the following methods.

a. PicOutputFlag may be set to pic_output_flag signaled via a sliceheader.

b. PicOutputFlag may be set to pic_output_flag signaled via a sliceheader when the corresponding layer is an output layer, whilePicOutputFlag may be set to 0 when the corresponding layer is not anoutput layer.

c. When the corresponding layer is not an output layer, a higher layerright above the layer is an IRAP picture, and LayerinitialisedFlag ofall lower reference layers of the higher layer is 1, PicOutputFlag maybe set to 0.

d. When the corresponding layer is not an output layer, a higher layerright above the layer is an IRAP picture, a lower reference layer havingLayerinitialisedFlag equal to 0 is present among lower reference layersof the higher layer, and an output layer is present among higher layersof the layer, PicOutputFlag of the layer may be set to pic_output_flagsignaled via a slice header. Here, if there is a layer withPicOutputFlag equal to 1 which is not an output layer among lower layersclose to the layer, PicOutputFlag of the layer may be set to 0.

e. When the corresponding layer is not an output layer, a picturecorresponding to an output layer is absent in the same AU, the layer isa direct reference picture of an output layer picture (a picture of thelayer directly referenced by the corresponding layer) or an indirectreference picture (a layer needing a layer directly referenced by thecorresponding layer for encoding/decoding), the picture is a direct orindirect reference picture of the output layer picture and is a highestpicture (that is, a picture with greatest nuh_layer_id) among decodedpictures with pic_output_flag equal to 1 signaled via a slice header,PicOutputFlag of the picture may be set to 1. Otherwise, PicOutputFlagmay be set to 0.

f. When the corresponding layer is not an output layer, a picturecorresponding to an output layer is absent in the same AU, the layer isa direct reference picture of an output layer picture (a picture of thelayer directly referenced by the corresponding layer) or an indirectreference picture (a layer needing a layer directly referenced by thecorresponding layer for encoding/decoding), and the picture is a director indirect reference picture of the output layer picture which hasLayerinitializedFlag equal to 1 and is a highest picture (that is, apicture having greatest nuh_layer_id) among decoded pictures withpic_output_flag equal to 1 signaled via a slice header, PicOutputFlag ofthe picture may be set to 1. Otherwise, PicOutputFlag may be set to 0.

In c, d, e and f, the decoding apparatus may control output of picturesin the DPB to achieve the same results as those from settingPicOutputFlag.

When it is determined whether to output a picture, the video decodingapparatus performs motion estimation and motion compensation on thecurrent encoding/decoding target picture using reference pictures in areference picture list (S960).

Here, motion estimation and motion compensation may be performed on thecurrent encoding/decoding target picture using reference pictures in thereference picture list by a general inter prediction method.

After inter prediction is performed using reference pictures, theparameters needed for decoding may be reset (S970). The video decodingapparatus may set FirstPicInLayerDecodedFlag[i] of the layer includingthe current encoding/decoding target picture, for example, a layer withnuh_layer_id=i, to 1 when FirstPicInLayerDecodedFlag[i] is 0.

The decoding apparatus may conduct the decoding method from S910 to S970by layers. When a decoding process for a first AU is finished, pictureswith PicOutputFlag equal to 1 among layers in the first AU may beoutput.

For instance, when pictures in base to nth layers are IRAP pictures, ifoutput layers to originally output are included in the base to nthlayers (0<=n), decoded pictures in the output layer are output. If theoutput layers are included in (n+1)th to highest layers, a decodedpicture in the nth layer may be output.

Subsequently, when an AU including an IRAP picture from (n+1)th to kthlayers is decoded in a process of decoding subsequent AUs, if outputlayers to originally output among decoded pictures in from 0th to kthlayers are included in base to kth layers, decoded pictures in theoutput layer are output. If the output layers are included in (k+1)th tohighest layers, a decoded picture in the kth layer or a picture withPicOutputFlag equal to 1 may be output.

The decoding apparatus may repeat the foregoing process on an AU until apicture in the highest layer is decoded, thereby decoding the bitstream.

FIG. 10 illustrates a method of setting decoding parameters according toan embodiment of the present invention.

When the first random access occurs in a bitstream, the decodingapparatus may set an initialization flag of a base layer to 1 and aninitialization flag of a higher layer than the base layer to 0 (S1010).

An initialization flag (LayerInitialisedFlag) indicates whether a layeramong layers included in a bistream is an IRAP picture and all referencelayers of the layer are normally initialized so that a decoded picturein the layer is available as an output from the decoding apparatus whenthe bitstream in which the base layer included in an AU at a randomaccess point is an IRAP picture (that is, IDR picture, BLA picture orCRA picture) is decoded at the random access point. That the referencelayers are normally initialized may mean that the picture is correctlydecoded and may output from the decoding apparatus.

The decoding apparatus may set an initialization flag indicating whethera decoded picture in a layer is available as an output from the decodingapparatus before decoding the bitstream. When a currentencoding/decoding target picture is an IRAP picture which is in the baselayer (nuh_layer_id=0) included in the AU at the random access point,the decoding apparatus may set LayerInitialisedFlag[0] to 1, and setLayerInitialisedFlag[i] of other layers (1<=i<=63) to 0.

After the initialization flags are set, the decoding apparatus may set aflag variable (NoRaslOutputFlag) indicating whether a picture is used asa random access point.

The decoding apparatus may set the flag variable to 1 when the targetpicture is an IDR picture or BLA picture, or an IRAP picture which is afirst picture of a layer (having the same nuh_layer_id as the layer ofthe current encoding/decoding target picture) in the bitstream. Thedecoding apparatus may set the flag variable to 1 when the targetpicture is an IRAP picture, an initialization flag of the layer of thetarget picture is 0, and initialization flags of all reference layers inthe layer of the target picture are 1 (S1020).

When the target picture is an IRAP picture and the flag variable of thetarget picture is determined to be 1, the decoding apparatus maydetermine whether the initialization flag of the target picture is 0 andthe initialization flags of all reference layers in the layer of thetarget picture are 1. When it is determined that the initialization flagof the target picture is 0 and the initialization flags of all referencelayers in the layer of the target picture are 1, the decoding apparatusmay set the initialization flag of the target layer from 0 to 1 (S1030).

That is, when a condition for the layer of the target picture to becorrectly decoded is satisfied, the initialization flag, which is set to0 in S1010, may be changed to 1.

When the initialization flag is set, the target picture is decoded as inthe method of FIG. 9, and accordingly pictures of higher layers may begradually output depending on whether decoded pictures are output.

FIG. 11 is a flowchart illustrating a method of decoding an inter-layervideo according to an embodiment of the present invention. FIG. 11illustrates a method of decoding pictures by each AU and applying suchdecoding to a plurality of layers to finally decode a bitstream.

When random access occurs, the decoding apparatus sets parameters fordecoding as in S910 of FIG. 9 and FIG. 10 and decodes pictures includedin a first AU (S1110). A process of decoding the pictures may correspondto S910 to S970 of FIG. 9, and thus description thereof is omittedherein to avoid redundancy.

When decoding the pictures in the first AU is finished, the decodingapparatus may determine whether to output the individual pictures in thefirst AU (S1120).

In determining whether to output the individual pictures, the decodingapparatus may determine not to output a decoding target picture when aninitialization flag of a target layer including the decoding targetpicture is 0.

Also, the decoding apparatus may determine to output the target picturewhen the target layer including the decoding target picture is not anoutput layer, no output target picture included in the output layer ispresent in an AU including the target layer, that is, the output targetpicture is determined not to be output, the target layer is used for theoutput layer for reference so that the target picture is used as areference picture of the output picture, and the target picture is ahighest picture among decoded pictures with flag information equal to 1,the flag information being signaled via a slice header and representingwhether to output a picture.

Further, the decoding apparatus may determine to output the targetpicture when the target layer including the decoding target picture isnot an output layer, no output target picture included in the outputlayer is present in an AU including the target layer, that is, theoutput target picture is determined not to be output, the target layeris used for the output layer for reference so that the target picture isused as a reference picture of the output picture, the target layer hasan initialization flag of 1, and the target picture is a highest pictureamong decoded pictures with flag information equal to 1, the flaginformation being signaled via a slice header and representing whetherto output a picture.

When a process of determining output of the first AU is finished, thedecoding apparatus may decode pictures included in a next AU, that is, asecond AU (S1130) and determine whether to output the individualpictures (S1140).

A method of determining whether to output the pictures in the second AUmay be the same as used for the first AU.

The decoding apparatus may output a picture determined to be outputcorresponding to an output time of each picture.

As described above, when random access occurs, the decoding apparatusdetermines whether to output pictures in an AU individually to outputoutputable pictures and subsequently determines whether to outputpictures in a next AU, thereby outputting pictures in layers possible tocorrectly decode and output. Accordingly, pictures of higher layers maybe gradually decoded and output.

According to the present invention, there are provided a method ofdecoding a bitstream from a random access point when an AU at the randomaccess point includes an IRAP picture and a non-IRAP picture, orincludes only IRAP pictures which have two or more different NAL unittypes, and an apparatus using the same.

In the aforementioned embodiments, methods have been described based onflowcharts as a series of steps or blocks, but the methods are notlimited to the order of the steps of the present invention and any stepmay occur in a step or an order different from or simultaneously as theaforementioned step or order. Further, it can be appreciated by thoseskilled in the art that steps shown in the flowcharts are not exclusiveand other steps may be included or one or more steps do not influencethe scope of the present invention and may be deleted.

The foregoing embodiments include various aspects of examples. Althoughall possible combinations to illustrate various aspects may notdescribed herein, it will be understood by those skilled in the art thatvarious combinations may be made therein without departing from thespirit and scope of the invention as defined by the appended claims.Therefore, all differences, changes and modifications within the scopewill be construed as being included in the present invention.

The invention claimed is:
 1. A method of decoding a bitstream comprisinga plurality of layers, the method comprising: setting parameters fordecoding the bitstream from a random access point when random accessoccurs; and decoding a decoding target picture, wherein the setting ofthe parameters comprises setting an initialization flag indicatingwhether the decoding target picture is correctly decoded and output whenthe random access occurs; setting a flag variable (NoRaslOutputFlag)indicating whether the decoding target picture is used as a randomaccess point; and resetting an initialization flag of a target layer to1 when the decoding target picture is an intra random access point(IRAP) picture, the flag variable is 1, the initialization flag of thedecoding target picture is 0 and initialization flags of all referencelayers of the target layer are 1, wherein the initialization flag equalto 0 indicates that the decoding target picture is not correctly decodedand output when the random access occurs and the initialization flagequal to 1 indicates that the decoding target picture is correctlydecoded and output when the random access occurs.
 2. The method of claim1, wherein the setting of the initialization flag sets theinitialization flag for a base layer of an access unit (AU) at which therandom access occurs to 1 and sets the initialization flag for a higherlayer than the base layer to
 0. 3. The method of claim 2, wherein thesetting of the flag variable (NoRaslOutputFlag) sets the flag variableto 1 when the decoding target picture is an instantaneous decodingrefresh (IDR) picture, a broken link access (BLA) picture or an intrarandom access point (IRAP) picture which is a first picture of thetarget layer of the bitstream.
 4. The method of claim 2, wherein thesetting of the flag variable (NoRaslOutputFlag) sets the flag variableto 1 when the target picture is an IRAP picture, the initialization flagof the target layer including the target picture is 0, and theinitialization flags of all reference layers of the target layerincluding the target picture are
 1. 5. The method of claim 1, furthercomprising calculating a picture order count (POC) of the decodingtarget picture as an identifier; constructing a reference picture setand marking a reference picture type for inter prediction of thedecoding target picture; determining whether to output the decodingtarget picture based on the initialization flag; and performing motionestimation and motion compensation on the decoding target picture usinga reference picture in a reference picture list generated based on thereference picture set.
 6. The method of claim 5, wherein the determiningof whether to output the decoding target picture determines not tooutput the decoding target picture when the decoding target picture isan IRAP picture and the initialization flag of the target layer is
 0. 7.The method of claim 5, wherein the determining of whether to output thedecoding target picture determines to output the decoding target picturewhen the target layer is not an output layer as an output target, anoutput target picture belonging to the output layer in an AU which thetarget layer belongs to is determined not to be output, the target layeris used as reference of the output layer so that the decoding targetpicture is used as a reference picture of the output target picture, andthe decoding target picture is a highest picture among decoded pictureswith flag information equal to 1, the flag information being signaledvia a slice header and representing whether to output a picture.
 8. Themethod of claim 5, wherein the determining of whether to output thedecoding target picture determines to output the decoding target picturewhen the target layer is not an output layer as an output target, anoutput target picture belonging to the output layer in an AU which thetarget layer belongs to is determined not to be output, the target layeris used as reference of the output layer so that the decoding targetpicture is used as a reference picture of the output target picture, theinitialization flag of the target layer is 1, and the decoding targetpicture is a highest picture among decoded pictures with flaginformation equal to 1, the flag information being signaled via a sliceheader and representing whether to output a picture.
 9. The method ofclaim 5, wherein the calculating of the POC sets the POC of the decodingtarget picture to 0 when a picture of a base layer in the same AU is anIDR picture, and sets a most significant bit (MSB) of the decodingtarget picture to 0 when the picture of the base layer is a BLA pictureor a first CRA picture of the bitstream.
 10. The method of claim 1,further comprising decoding pictures comprised in a first AUcorresponding to the random access point; determining whether to outputthe individual pictures in the first AU; decoding pictures comprised ina second AU following the first AU; and determining whether to outputthe individual pictures in the second AU.
 11. The method of claim 10,further comprising determining outputting a first AU picture whichoutputs a picture determined to be output among the pictures in thefirst AU, wherein the outputting of the first AU picture outputs anoutput picture of an output layer when the bitstream comprises a baselayer to a highest layer, all pictures from the base layer to an nthlayer (0≤n) are IRAP pictures, and the base layer to the nth layercomprise the output layer, and outputs a decoded picture in the nthlayer when an (n+1)th layer to the highest layer comprise the outputlayer.
 12. The method of claim 11, further comprising determiningoutputting a second AU picture which outputs a picture determined to beoutput among the pictures in the second AU, wherein the outputting ofthe second AU picture outputs an output picture of an output layer whenthe (n+1)th layer to a kth layer (n+1≤k) comprised in the second AUcomprise an IRAP picture and the base layer to the kth layer comprisethe output layer, and outputs a decoded picture in the kth layer or thepicture determined to be output when a (k+1) layer to the highest layercomprises the output layer.